Ozonolysis of carbon nanotubes

ABSTRACT

Methods of treating single walled and multiwalled carbon nanotubes with ozone are provided. The carbon nanotubes are treated by contacting the carbon nanotubes with ozone at a temperature range between 0° C. and 100° C. to yield functionalized nanotubes which are greater in weight than the untreated carbon nanotubes. The carbon nanotubes treated according to methods of the invention can be used to prepare complex structures such as three dimensional networks or rigid porous structures which can be utilized to form electrodes for fabrication of improved electrochemical capacitors. Useful catalyst supports are prepared by contacting carbon nanotube structures such as carbon nanotube aggregates, three dimensional networks or rigid porous structures with ozone in the temperature range between 0° C. and 100° C.

CROSS REFERENCE INFORMATION

This application claims priority to and the benefit of U.S. ProvisionalApplication Ser. No. 60/720,806, filed Sep. 26, 2005, and of U.S.Provisional Application Ser. No. 60/621,132, filed Oct. 22, 2004, all ofwhich are hereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to methods of treating the surface of singlewalled and multiwalled carbon nanotubes with ozone. The invention alsoencompasses methods of making aggregates of ozone-treated nanotubes, andmethods of using the same. The invention further relates to methods ofmaking complex structures such as three dimensional networks or rigidporous structures comprised of such ozone-treated carbon nanotubeslinked to one another. The invention also includes methods of making acatalyst support from aggregates, three dimensional networks, or rigidporous structures that have been treated with ozone.

2. Description of the Related Art

Carbon Nanotubes

This invention lies in the field of submicron graphitic carbon fibrils,commonly referred to as nanotubes. Carbon fibrils are vermicular carbondeposits having diameters less than 1.0μ, preferably less than 0.5μ, andeven more preferably less than 0.2μ. Carbon nanotubes can be eithermulti walled (i.e., have more than one graphite layer on the nanotubeaxis) or single walled (i.e., have only a single graphite layer on thenanotube axis). Other types of carbon nanotubes are also describedbelow.

The carbon nanotubes which can be treated as taught in this application,are distinguishable from commercially available continuous carbonfibers. In contrast to these fibers which have aspect ratios (L/D) of atleast 10⁴ and often 10⁶ or more, carbon fibrils have desirably large,but unavoidably finite, aspect ratios. The diameter of continuous fibersis also far larger than that of fibrils, being always >1.0μ andtypically 5 to 7μ.

Carbon nanotubes differ physically and chemically from continuous carbonfibers which are commercially available as reinforcement materials, andfrom other forms of carbon such as standard graphite and carbon black.Standard graphite, because of its structure, can undergo oxidation toalmost complete saturation. Moreover, carbon black is amorphous carbongenerally in the form of spheroidal particles having a graphenestructure, carbon layers around a disordered nucleus. The differencesmake graphite and carbon black poor predictors of carbon nanotubechemistry.

Carbon nanotubes exist in a variety of forms and have been preparedthrough the catalytic decomposition of various carbon-containing gasesat metal surfaces. Such vermicular carbon deposits have been observedalmost since the advent of electron microscopy. (Baker and Harris,Chemistry and Physics of Carbon, Walker and Thrower ed., Vol. 14, 1978,p. 83; Rodriguez, N., J. Mater. Research, Vol. 8, p. 3233 (1993)).

In 1976, Endo et al. (see Oberlin, A. and Endo, M., J. of CrystalGrowth, Vol. 32 (1976), pp. 335-349), hereby incorporated by reference,elucidated the basic mechanism by which such carbon fibrils grow. Theywere seen to originate from a metal catalyst particle, which, in thepresence of a hydrocarbon containing gas, becomes supersaturated incarbon. A cylindrical ordered graphitic core is extruded whichimmediately, according to Endo et al., becomes coated with an outerlayer of pyrolytically deposited graphite. These fibrils with apyrolytic overcoat typically have diameters in excess of 0.1μ, moretypically 0.2 to 0.5μ.

In 1983, Tennent, U.S. Pat. No. 4,663,230, hereby incorporated byreference, describes carbon fibrils that are free of a continuousthermal carbon overcoat and have multiple graphitic outer layers thatare substantially parallel to the fibril axis. As such they may becharacterized as having their c-axes, the axes which are perpendicularto the tangents of the curved layers of graphite, substantiallyperpendicular to their cylindrical axes. They generally have diametersno greater than 0.1μ and length to diameter ratios of at least 5.Desirably they are substantially free of a continuous thermal carbonovercoat, i.e., pyrolytically deposited carbon resulting from thermalcracking of the gas feed used to prepare them. Thus, the Tennentinvention provided access to smaller diameter fibrils, typically 35 to700 Å (0.0035 to 0.070μ) and to an ordered, “as grown” graphiticsurface. Fibrillar carbons of less perfect structure, but also without apyrolytic carbon outer layer have also been grown.

Tennent, et al., U.S. Pat. No. 5,171,560, hereby incorporated byreference, describes carbon fibrils free of thermal overcoat and havinggraphitic layers substantially parallel to the fibril axes such that theprojection of said layers on said fibril axes extends for a distance ofat least two fibril diameters. Typically, such fibrils are substantiallycylindrical, graphitic nanotubes of substantially constant diameter andcomprise cylindrical graphitic sheets whose c-axes are substantiallyperpendicular to their cylindrical axis. They are substantially free ofpyrolytically deposited carbon, have a diameter less than 0.1μ andlength to diameter ratio of greater than 5. These fibrils can beoxidized by the methods of the invention.

When the projection of the graphitic layers on the nanotube axis extendsfor a distance of less than two nanotube diameters, the carbon planes ofthe graphitic nanotube, in cross section, take on a herring boneappearance. These are termed fishbone fibrils. Geus, U.S. Pat. No.4,855,091, hereby incorporated by reference, provides a procedure forpreparation of fishbone fibrils substantially free of a pyrolyticovercoat. These carbon nanotubes are also useful in the practice of theinvention.

Carbon nanotubes of a morphology similar to the catalytically grownfibrils described above have been grown in a high temperature carbon arc(Iijima, Nature 354, 56, 1991). It is now generally accepted (Weaver,Science 265, 1994; de Heer, Walt A., “Nanotubes and the Pursuit ofApplications,” MRS Bulletin, April, 2004, both incorporated by referenceherein) that these arc-grown nanofibers have the same morphology as theearlier catalytically grown fibrils of Tennent. Arc grown carbonnanofibers often colloquially referred to as “bucky tubes”, are alsouseful in the invention.

Useful single walled carbon nanotubes and process for making them aredisclosed, for example, in “Single-shell carbon nanotubes of 1-nmdiameter”, S Iijima and T Ichihashi Nature, vol. 363, p. 603 (1993) and“Cobalt-catalysed growth of carbon nanotubes with single-atomic-layerwalls,” D S Bethune, C H Kiang, M S DeVries, G Gorman, R Savoy and RBeyers Nature, vol. 363, p. 605 (1993), both articles of which arehereby incorporated by reference.

Single walled carbon nanotubes are also disclosed in U.S. Pat. No.6,221,330 to Moy et. al., the contents therein of which are herebyincorporated by reference. Moy disclosed a process for producing hollow,single-walled carbon nanotubes by catalytic decomposition of one or moregaseous carbon compounds by first forming a gas phase mixture carbonfeed stock gas comprising one or more gaseous carbon compounds, eachhaving one to six carbon atoms and only H, O, N, S or Cl as heteroatoms, optionally admixed with hydrogen, and a gas phase metalcontaining compound which is unstable under reaction conditions for saiddecomposition, and which forms a metal containing catalyst which acts asa decomposition catalyst under reaction conditions; and then conductingsaid decomposition reaction under decomposition reaction conditions,thereby producing said nanotubes. The invention relates to a gas phasereaction in which a gas phase metal containing compound is introducedinto a reaction mixture also containing a gaseous carbon source. Thecarbon source is typically a C₁ through C₆ compound having as heteroatoms H, O, N, S or Cl, optionally mixed with hydrogen. Carbon monoxideor carbon monoxide and hydrogen is a preferred carbon feedstock.Increased reaction zone temperatures of approximately 400° C. to 1300°C. and pressures of between about 0 and about 100 p.s.i.g., are believedto cause decomposition of the gas phase metal containing compound to ametal containing catalyst. Decomposition may be to the atomic metal orto a partially decomposed intermediate species. The metal containingcatalysts (1) catalyze CO decomposition and (2) catalyze SWNT formation.

The invention of U.S. Pat. No. 6,221,330 may in some embodiments employan aerosol technique in which aerosols of metal containing catalysts areintroduced into the reaction mixture. An advantage of an aerosol methodfor producing SWNT is that it will be possible to produce catalystparticles of uniform size and scale such a method for efficient andcontinuous commercial or industrial production. The previously discussedelectric arc discharge and laser deposition methods cannot economicallybe scaled up for such commercial or industrial production. Examples ofmetal containing compounds useful in the invention include metalcarbonyls, metal acetyl acetonates, and other materials which underdecomposition conditions can be introduced as a vapor which decomposesto form an unsupported metal catalyst. Catalytically active metalsinclude Fe, Co, Mn, Ni and Mo. Molybdenum carbonyls and iron carbonylsare the preferred metal containing compounds which can be decomposedunder reaction conditions to form vapor phase catalyst. Solid forms ofthese metal carbonyls may be delivered to a pretreatment zone where theyare vaporized, thereby becoming the vapor phase precursor of thecatalyst. It was found that two methods may be employed to form SWNT onunsupported catalysts.

The first method is the direct injection of volatile catalyst. Thedirect injection method is described is U.S. Pat. No. 6,696,387,incorporated herein by reference. Direct injection of volatile catalystprecursors has been found to result in the formation of SWNT usingmolybdenum hexacarbonyl [Mo(CO)₆] and dicobalt octacarbonyl [CO₂(CO)₈]catalysts. Both materials are solids at room temperature, but sublime atambient or near-ambient temperatures—the molybdenum compound isthermally stable to at least 150°, the cobalt compound sublimes withdecomposition “Organic Syntheses via Metal Carbonyls,” Vol. 1, I. Wenderand P. Pino, eds., Interscience Publishers, New York, 1968, p. 40).

The second method described in U.S. Pat. No. 6,221,330 uses a vaporizerto introduce the metal containing compound (see FIG. 1 in U.S. Pat. No.6,221,330). In one preferred embodiment of the invention, the vaporizer10, shown at FIG. 2 of U.S. Pat. No. 6,221,330, comprises a quartzthermowell 20 having a seal 24 about 1″ from its bottom to form a secondcompartment. This compartment has two ¼″ holes 26 which are open andexposed to the reactant gases. The catalyst is placed into thiscompartment, and then vaporized at any desired temperature using avaporizer furnace 32. This furnace is controlled using a firstthermocouple 22. A metal containing compound, preferably a metalcarbonyl, is vaporized at a temperature below its decomposition point,reactant gases CO or CO/H₂ sweep the precursor into the reaction zone34, which is controlled separately by a reaction zone furnace 38 andsecond thermocouple 42. Although applicants do not wish to be limited toa particular theory of operability, it is believed that at the reactortemperature, the metal containing compound is decomposed eitherpartially to an intermediate species or completely to metal atoms. Theseintermediate species and/or metal atoms coalesce to larger aggregateparticles which are the actual catalyst. The particle then grows to thecorrect size to both catalyze the decomposition of CO and promote SWNTgrowth. In the apparatus of FIG. 1 of U.S. Pat. No. 6,221,330, thecatalyst particles and the resultant carbon forms are collected on thequartz wool plug 36. Rate of growth of the particles depends on theconcentration of the gas phase metal containing intermediate species.This concentration is determined by the vapor pressure (and thereforethe temperature) in the vaporizer. If the concentration is too high,particle growth is too rapid, and structures other than SWNT are grown(e.g., MWNT, amorphous carbon, onions, etc.) All of the contents of U.S.Pat. No. 6,221,330, including the Examples described therein, are herebyincorporated by reference.

U.S. Pat. No. 5,424,054 to Bethune et al., hereby incorporated byreference, describes a process for producing single-walled carbonnanotubes by contacting carbon vapor with cobalt catalyst. The carbonvapor is produced by electric arc heating of solid carbon, which can beamorphous carbon, graphite, activated or decolorizing carbon or mixturesthereof. Other techniques of carbon heating are discussed, for instancelaser heating, electron beam heating and RF induction heating.

Smalley (Guo, T., Nikoleev, P., Thess, A., Colbert, D. T., and Smally,R. E., Chem. Phys. Lett. 243: 1-12 (1995)), hereby incorporated byreference, describes a method of producing single-walled carbonnanotubes wherein graphite rods and a transition metal aresimultaneously vaporized by a high-temperature laser.

Smalley (Thess, A., Lee, R., Nikolaev, P., Dai, H., Petit, P., Robert,J., Xu, C., Lee, Y. H., Kim, S. G., Rinzler, A. G., Colbert, D. T.,Scuseria, G. E., Tonarek, D., Fischer, J. E., and Smalley, R. E.,Science, 273: 483-487 (1996)), hereby incorporated by reference, alsodescribes a process for production of single-walled carbon nanotubes inwhich a graphite rod containing a small amount of transition metal islaser vaporized in an oven at about 1200° C. Single-wall nanotubes werereported to be produced in yields of more than 70%.

Supported metal catalysts for formation of SWNT are also known. Smalley(Dai., H., Rinzler, A. G., Nikolaev, P., Thess, A., Colbert, D. T., andSmalley, R. E., Chem. Phys. Lett. 260: 471-475 (1996)), herebyincorporated by reference, describes supported Co, Ni and Mo catalystsfor growth of both multiwalled nanotubes and single-walled nanotubesfrom CO, and a proposed mechanism for their formation.

U.S. Pat. No. 6,761,870 (also WO 00/26138) to Smalley, et. al, herebyincorporated by reference, discloses a process of supplying highpressure (e.g., 30 atmospheres) CO that has been preheated (e.g., toabout 1000° C.) and a catalyst precursor gas (e.g., Fe(CO)₅) in CO thatis kept below the catalyst precursor decomposition temperature to amixing zone. In this mixing zone, the catalyst precursor is rapidlyheated to a temperature that is reported to result in (1) precursordecomposition, (2) formation of active catalyst metal atom clusters ofthe appropriate size, and (3) favorable growth of SWNTs on the catalystclusters.

Other methods of producing carbon nanotubes are disclosed in Resasco, etal., “Controlled production of single-wall carbon nanotubes by catalyticdecomposition of CO on bimetallic Co—Mo catalysts,” Chemical PhysicsLetters, 317 (2000) 497-503 and U.S. Pat. No. 6,333,016 to Resasco, et.all, both of which are hereby incorporated by reference. The carbonnanotubes are produced therein by contacting a carbon containing gaswith metallic catalytic particles.

Aggregates of Carbon Nanotubes and Assemblages

As produced, carbon nanotubes may be in the form of discrete nanotubes,aggregates of nanotubes or both.

Nanotubes are prepared as aggregates having various morphologies (asdetermined by scanning electron microscopy) in which they are randomlyentangled with each other to form entangled balls of nanotubesresembling bird nests (“BN”); or as aggregates consisting of bundles ofstraight to slightly bent or kinked carbon nanotubes havingsubstantially the same relative orientation, and having the appearanceof combed yarn (“CY”) e.g., the longitudinal axis of each nanotube(despite individual bends or kinks) extends in the same direction asthat of the surrounding nanotubes in the bundles; or, as, aggregatesconsisting of straight to slightly bent or kinked nanotubes which areloosely entangled with each other to form an “open net” (“ON”)structure. In open net structures the extent of nanotube entanglement isgreater than observed in the combed yarn aggregates (in which theindividual nanotubes have substantially the same relative orientation)but less than that of bird nest. Other useful aggregate structuresinclude the cotton candy (“CC”) structure.

The morphology of the aggregate is controlled by the choice of catalystsupport. Spherical supports grow nanotubes in all directions leading tothe formation of bird nest aggregates. Combed yarn and open nestaggregates are prepared using supports having one or more readilycleavable planar surfaces, e.g., an iron or iron-containing metalcatalyst particle deposited on a support material having one or morereadily cleavable surfaces and a surface area of at least 1 squaremeters per gram. U.S. Pat. No. 6,143,689 to Moy et al., entitled“Improved Methods and Catalysts for the Manufacture of Carbon Fibrils”,filed Jun. 6, 1995, hereby incorporated by reference, describesnanotubes prepared as aggregates having various morphologies (asdetermined by scanning electron microscopy).

Further details regarding the formation of carbon nanotube aggregatesmay be found in the disclosure of U.S. Pat. No. 5,165,909 to Tennent;U.S. Pat. No. 5,456,897 to Moy et al.; Snyder et al., U.S. Pat. No.5,707,916 , filed May 1, 1991, and PCT Application No. US89/00322, filedJan. 28, 1989 (“Carbon Fibrils”) WO 89/07163, and Moy et al., U.S. Pat.No. 5,456,897 filed Aug. 2, 1994 and PCT Application No. US90/05498,filed Sep. 27, 1990 (“Battery”) WO 91/05089, and U.S. Pat. No. 5,500,200to Mandeville et al., filed Jun. 7, 1995 and U.S. Pat. No. 5,456,897filed Aug. 2, 1994 and U.S. Pat. No. 5,569,635 filed Oct. 11, 1994 byMoy et al., all of which are assigned to the same assignee as theinvention here and are hereby incorporated by reference.

Nanotube mats or assemblages have been prepared by dispersing carbonnanotubes in aqueous or organic mediums and then filtering the nanotubesto form a mat or assemblage. The mats have also been prepared by forminga gel or paste of nanotubes in a fluid, e.g. an organic solvent such aspropane and then heating the gel or paste to a temperature above thecritical temperature of the medium, removing the supercritical fluid andfinally removing the resultant porous mat or plug from the vessel inwhich the process has been carried out. See, U.S. patent applicationSer. No. 08/428,496 entitled “Three-Dimensional Macroscopic Assemblagesof Randomly Oriented Carbon Fibrils and Composites Containing Same” byTennent et al, which has issued as U.S. Pat. No. 5,691,054 on Nov. 25,1997, hereby incorporated by reference.

Oxidation of Carbon Nanotubes

While many uses have been found for carbon nanotubes and aggregates ofcarbon nanotubes, as described in the patents and patent applicationsreferred to above, it had been previously discovered that many differentand important uses may still be developed if the nanotubes surfaces arefunctionalized with oxygen containing moieties. One method used tofunctionalizing the carbon nanotubes is through oxidation reactions withliquid oxidizing agents such as nitric acid or hydrogen peroxide.Oxidation can permit interaction of the functionalized nanotubes withvarious substrates to form unique compositions of matter with uniqueproperties and permit structures of carbon nanotubes to be created basedon linkages between the functional sites on the surfaces of the carbonnanotubes.

However, a common unwanted side effect is the simultaneous destructionof the carbon nanotubes or carbon nanotube structures themselves. Andthus, not only is the strength and integrity of the carbon nanotube orcarbon nanotube structure compromised, but there is also a restrictedlimit as to how much oxygen containing moieties can be deposited ontothe carbon nanotube or carbon nanotube structure. As such, there is aneed for an improved method for functionalizing carbon nanotubes whichwill result in less destruction to the carbon nanotube or carbonnanotube structure while at the same time yielding a higherconcentration of oxygen containing moieties on the carbon nanotubesurface.

SUMMARY OF THE INVENTION

The present invention provides methods of treating single walled ormultiwalled carbon nanotubes with ozone at a temperature range of from0° C. to 100° C. under conditions sufficient to form functionalizednanotubes which are greater in weight than the original carbonnanotubes. Preferably, the nanotubes are contacted with ozone at orabout room temperature. More preferably, the nanotubes are contactedwith ozone at a temperature range between 0° C. and 60° C. and mostpreferably between 20° C. and 50° C.

The ozone may be contacted with the carbon nanotubes via a gaseous orliquid medium. The carbon nanotubes to be treated may be in individualform or in the form of carbon nanotube structures such as aggregateshaving a macromorpology resembling the shape of a cotton candy, birdnest, combed yarn or open net. Other carbon nanotube structures includemats, assemblages, three dimensional networks, rigid porous structures,etc. Preferred multiwalled carbon nanotubes have diameters no greaterthan 1 micron and preferred single walled carbon nanotubes havediameters less than 5 nm.

The ozone treated carbon nanotubes can be further subjected to asecondary treatment step whereby the oxygen containing moieties of theozone treated nanotubes react with suitable reactants to add at least asecondary group onto the surface of the treated nanotubes.

The ozone treated carbon nanotubes are also useful in preparing anetwork of carbon nanotubes, a rigid porous structure or as startingmaterial for electrodes utilized in electrochemical capacitors.

Catalyst supports are preferably prepared by contacting or treatingcarbon nanotube aggregates, three dimensional networks or rigid porousstructures with ozone at a temperature range of from 0° C. to 100° C.Preferably, the aggregates, three dimensional networks or rigid porousstructures are contacted with ozone at or about room temperature. Morepreferably, the aggregates, three dimensional networks or rigid porousstructures are contacted with ozone at a temperature range between 0° C.and 60° C. and most preferably between 20° C. and 50° C. Catalystsupport structures functionalized using ozone exhibit higher acid titer,thus enabling higher catalyst loading and better retention of theiroriginal support structure and integrity.

Electrochemical capacitors assembled from electrodes made from the ozonetreated carbon nanotubes of the invention exhibit enhancedelectrochemical characteristics, such as specific capacitance.

In summary, the present invention includes a carbon nanotube structurecomprising a multiplicity of carbon nanotubes entangled with oneanother, said structure exhibiting upon titration an acid titer from 1to 2 meq/g. The carbon nanotube structure may be in the form ofaggregate of carbon nanotubes having a macromorphology resembling ashape selected from the group consisting of cotton candy, bird nests,combed yam and open net aggregates. The carbon nanotube structure mayalso be a three dimensional network, a carbon nanotube mat orassemblage, a rigid porous structure, or any other carbon nanotubestructure.

The present invention further includes carbon nanotubes which exhibitupon titration an acid titer between 1.6 and 2.2 meq/g, or greater than2 meq/g, or between 2.5 to 3.5 meq/g. Additionally, the presentinvention includes ozone treated carbon nanotubes which exhibit upontitration an acid titer increase of at least 1.5 meq/g, or of at least 2meq/g, or between 1.5 meq/g and 3 meq/g, when compared to the acid titerof the non-ozone treated carbon nanotubes.

Other improvements which the present invention provides over the priorart will be identified as a result of the following description whichsets forth the preferred embodiments of the present invention. Thedescription is not in any way intended to limit the scope of the presentinvention, but rather only to provide a working example of the presentpreferred embodiments. The scope of the present invention will bepointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of fibril weight as a function of reaction time duringozone treatment based on the results of Example 2.

FIG. 2 displays several oxygen Is spectra of the various fibril samplesin accordance with Example 5.

FIG. 3 illustrates electron micrographs of ozone treated fibrils inaccordance with Example 6.

FIGS. 4A and 4B are TEM micrographs illustrating fibrils before andafter treatment with ozone.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Definitions

The terms “nanotube”, “nanofiber” and “fibril” are used interchangeablyto refer to single walled or multiwalled carbon nanotubes. Each refersto an elongated structure preferably having a cross section (e.g.,angular fibers having edges) or a diameter (e.g., rounded) less than 1micron (for multiwalled nanotubes) or less than 5 nm (for single wallednanotubes). The term “nanotube” also includes “buckytubes”, and fishbonefibrils.

“Multiwalled nanotubes” as used herein refers to carbon nanotubes whichare substantially cylindrical, graphitic nanotubes of substantiallyconstant diameter and comprise cylindrical graphitic sheets or layerswhose c-axes are substantially perpendicular to the cylindrical axis,such as those described, e.g., in U.S. Pat. No. 5,171,560 to Tennent, etal.

“Single walled nanotubes” as used herein refers to carbon nanotubeswhich are substantially cylindrical, graphitic nanotubes ofsubstantially constant diameter and comprise a single cylindricalgraphitic sheet or layer whose c-axis is substantially perpendicular totheir cylindrical axis, such as those described, e.g., in U.S. Pat. No.6,221,330 to Moy, et al.

The term “functional group” refers to groups of atoms that give thecompound or substance to which they are linked characteristic chemicaland physical properties.

A “functionalized” surface refers to a carbon surface on which chemicalgroups are adsorbed or chemically attached.

“Graphenic” carbon is a form of carbon whose carbon atoms are eachlinked to three other carbon atoms in an essentially planar layerforming hexagonal fused rings. The layers are platelets only a few ringsin diameter or they may be ribbons, many rings long but only a few ringswide.

“Graphitic” carbon consists of graphenic layers which are essentiallyparallel to one another and no more than 3.6 angstroms apart.

The term “aggregate” refers to a dense, microscopic particulatestructure comprising entangled carbon nanotubes.

The term “micropore” refers to a pore which has a diameter of less than2 nanometers.

The term “mesopore” refers to pores having a cross section greater than2 nanometers and less than 50 nanometers.

The term “surface area” refers to the total surface area of a substancemeasurable by the BET technique.

The term “accessible surface area” refers to that surface area notattributed to micropores (i.e., pores having diameters or cross-sectionsless than 2 nm).

The term “isotropic” means that all measurements of a physical propertywithin a plane or volume of the structure, independent of the directionof the measurement, are of a constant value. It is understood thatmeasurements of such non-solid compositions must be taken on arepresentative sample of the structure so that the average value of thevoid spaces is taken into account.

The term “untreated” when used in comparison to “ozone treated” carbonnanotubes, aggregates or any other carbon nanotube structures mean thatthat the carbon nanotubes, aggregates, or structures have not beenspecifically treated with ozone. It does not preclude carbon nanotubes,aggregates, or structures which have been subjected to other non-ozonetreatments before the treatment with ozone.

Methods of Treating Carbon Nanotubes with Ozone

In the present invention, the surface properties and characteristics ofcarbon nanotubes are modified by subjecting them to treatments withozone in accordance with the preferred embodiment. One desirable resultof treating the fibrils in accordance with the preferred embodiment isthat the surface of the fibrils become functionalized with oxygencontaining moieties.

Additionally, the resulting ozone treated nanotubes can be easilydispersed in both organic and inorganic solvents, and especially inwater. The ozone treated nanotubes can be placed in matrices of othermaterials, such as plastics, or made into structures useful incatalysis, chromatography, filtration systems, electrodes, capacitorsand the like. Furthermore, the ozone treated nanotubes can also be usedto form other useful carbon nanotube structures as discussed in thesubsequent sections.

In the preferred embodiment, the carbon nanotubes are substantiallycylindrical, graphitic carbon fibrils of substantially constant diameterand are substantially free of pyrolytically deposited carbon. Thenanotubes include those having a length to diameter ratio of greaterthan 5 with the projection of the graphite layers on the nanotubesextending for a distance of at least two nanotube diameters.

The single walled and multiwalled carbon nanotubes useful for themethods of the present invention have been more specifically describedunder the previous heading “Carbon Nanotubes.” In a preferredembodiment, the multiwalled nanotubes are prepared according to U.S.Pat. No. 5,171,560 to Tennent, et al. or U.S. Pat. No. 6,696,387 to Moy,et. al, both of which are incorporated herein by reference. Themultiwalled carbon nanotubes preferably have diameters no greater thanone micron, more preferably no greater than 0.2 micron. Even morepreferred are multiwalled carbon nanotubes having diameters between 2and 100 nanometers, inclusive; most preferably between 3.5 and 75nanometers.

Alternatively, preferred single walled carbon nanotubes are prepared asdisclosed in U.S. Pat. No. 6,211,330 to Moy, et. al, incorporated hereinby reference. The single walled carbon nanotubes preferably havediameters no greater than 5 nanometers, more preferably between 0.6 and5 nanometers.

Ozone used to treat the carbon nanotubes may be delivered in the form ofgaseous ozone, liquid ozone, or ozone dissolved in aqueous solvent suchas water. Ozone containing gas may be optionally diluted with gases suchas oxygen, air, nitrogen, noble gases and mixtures thereof. Anyconventionally or commercially available ozone generator may be used toproduce the ozone or ozone containing gas. The ozone generator may befed with gases such as air or pure oxygen in order to generate the ozoneor ozone containing gas. The use of air as the feed gas will often yieldan ozone containing gas which also contains oxides of nitrogen that mayreact with a medium such as water to create nitric acid. This extraoxides of nitrogen or nitric acid byproduct in the ozone containing gasmay affect the desired functionalization of the carbon nanotube. On theother hand, the use of pure oxygen as the feed gas will yield a purerozone containing gas, without any or with much smaller amounts of extraby products such as oxides of nitrogen which may affect the desiredfunctionalization of the carbon nanotubes.

In one embodiment, ozone may be introduced to the carbon nanotubes at arate of 250 mg/hr. Preferably, ozone is introduced to the carbonnanotubes at a rate of 200 to 300 mg/hr. One skilled in the art willunderstand that the actual flow rate of the ozone containing gas willdepend on the amount of ozone in the gas and the quantity of carbonnanotubes to treat.

The ozone may be introduced to the carbon nanotubes using any knownconventional reactors, processes or methods, including through verticalreactors such as through the use of a vertical tube reactor, packed bed,fluidized bed, etc. Non-vertical or horizontal reactors may also beutilized. For example, ozone may be fed through a sparger into atumbling reactor or rotating drum reactor wherein the vessel housing thecarbon nanotubes is rotated to more evenly distribute the carbonnanotube's exposure to the ozone and result in more evenfunctionalization of the carbon nanotubes.

In the preferred embodiment, carbon nanotubes are treated with ozone attemperatures ranging from 0° C. to 100° C. Preferably, the nanotubes arecontacted with ozone at or about room temperature. More preferably, thenanotubes are contacted with ozone at a temperature range between 0° C.and 60° C. and most preferably between 20° C. and 50° C.

In the exemplary embodiment, this ozonolysis process at the preferredtemperatures may be carried on for over 24 hours. Preferably, thereaction is permitted to continue between 3 to 8 hours, or 10 to 45hours, and more preferably between 15 to 25 hours. One skilled in theart will understand that the run time of this process depend on factorssuch as the size and temperature of the reactor, the amount of carbonfibrils needed for treatment, the rate of ozone being introduced to thefibrils, the manner by which ozone is introduced to the fibrils (i.e, ina gaseous or liquid medium), the desired acidity, etc.

Carbon nanotubes treated in accordance with this preferred embodimentresult in a number of unexpected benefits.

It has been unexpectedly found that the ozone treated carbon nanotubesof the preferred embodiment experience a weight gain instead of a weightloss which is what would have been expected when treating carbonnanotubes with oxidizing agents. The increase in weight may be greaterthan 1%, and preferably greater than 5%. In one embodiment, the ozonetreated carbon nanotubes of the preferred embodiment exhibit a 5 to 20%weight gain, more preferably a 10 to 15% weight gain.

Without being bound to any theory, it is believed this overall weightgain can be attributed to the ozone at the recited temperature rangespreferentially engaging in surface treatment of the carbon nanotubesurface (thereby resulting in the attachment, formation, substitution ordeposition of functional groups onto the surface of the carbon nanotubesand aggregates) as opposed to engaging in uncapping, stripping orshortening reactions (thereby resulting in either minimal or nomeasurable or visual loss of carbon mass) that are more typical withother oxidizing agents at higher temperatures (for example nitricacid—degrades aggregates to form weathered rope structures). On a moreatomic level, ozone treatment at moderate temperatures in accordancewith the preferred embodiment is believed to proceed via an additionreaction by which ozone forms a live membered “ozonide” ring with thedouble bonded surface carbon atoms which then decomposes into COOH andC═O. This result is surprising since ozone is a strong oxidant at highertemperatures, and thus would have been expected to aggressively uncap,strip or shorten the carbon nanotubes as with other strong oxidants suchas nitric acid. The mechanisms of ozonolysis is described in Murray,Robert W., “The Mechanism of Ozonolysis,” Accounts of Chemical Research,Volume 1, pp. 313-320 (October, 1968), herein incorporated by reference.

Consistent with the weight gain, it has also been unexpectedly foundthat treatment of carbon nanotubes with ozone in accordance with thepreferred embodiment yields carbon nanotubes with higher amounts offunctional groups (especially acidic groups) on the nanotube surfacethan treatments using other oxidizing agents. (As a result, in thespecification and claims, the term “functionalized” nanotubes may beused interchangably with “ozone treated carbon nanotubes” where thenanotubes were treated with ozone). Specifically, carbon nanotubestreated with ozone can reach a higher acid titer (i.e., have more acidicgroups) than those treated with other oxidizing agents such as nitricacid and hydrogen peroxide. Acid titer can be measured using the methodsdescribed in the application, such as the method described under theheading entitled “Method of Measuring Titer” in the Examples section.

Ozone treated carbon nanotubes of the preferred embodiment can reach anacid titer greater than 2 meq/g, preferably in the range of 1.6 to 2.2meq/g. Alternatively, ozone treated carbon nanotubes can reach an acidtiter in the range of 0.0040-0.0080 meq/m². As acid titer measurementsare an indication of the quantity of acid reacting oxygen containingmoieties such as COOH, phenolic OH, lactone, etc. which have beendeposited onto the carbon nanotubes, the exceedingly high acid titerreached confirms that the ozone in the method of the preferredembodiment is reacting with the surface side walls of the carbonnanotubes and not just the end caps.

It has also been found that ozone can be used in accordance with thepreferred embodiment to further treat carbon nanotubes which havealready been oxidized or functionalized by other oxidizing agents. Forexample, a single or multi walled carbon nanotube mixture which containamorphous carbon can first be oxidized with nitric acid usingconventional methods to, inter alia, purify the mixture (i.e., to removethe amorphous carbon via oxidation). These oxidized carbon nanotubeswill be consequently functionalized with some oxygen containingmoieties, and thus will yield a low acid titer upon titration. Whenfurther treating these oxidized carbon nanotubes with ozone inaccordance with the preferred embodiment, the ozone treated carbonnanotubes was discovered to exhibit upon titration an increase in acidtiter by at least 1.5 meq/g and preferably greater than 2 meq/g. Forexample, the increase in acid titer may be between 1.5 meq/g to 3 meq/g,or between 2 meq/g to 2.5 meq/g. (These acid titer increases may alsoapply to ozone treated carbon nanotubes when comparing to as made carbonnanotubes which have not previously been treated with oxidizing agents).The final ozone treated carbon nanotubes may themselves exhibit upon atitration an acid titer greater than 2.5 meq/g or between 2.5 meq/g to3.5 meq/g.

Additionally, the use of ozone to treat carbon nanotubes at the recitedtemperature range also results in a more efficient generation (higherpercentage) of surface acidic groups compared to carbon nanotubes whichhave been oxidized with nitric acid or hydrogen peroxide. Carbonnanotubes treated with ozone at the recited temperature range havehigher oxygen content than those oxidized with nitric acid or hydrogenperoxide. The surface of the ozone treated carbon nanotubes of thepreferred embodiment can have an oxygen content greater than 4 percent(e.g., 4-10%), preferably greater than 6 percent (e.g., 6-10%).Preferred functional groups resulting from the ozone treatment includecarboxyl, anhydride and ketone.

It has been further discovered that ozone treatment of carbon nanotubesin accordance with the preferred embodiment also results in lessdestruction to the carbon nanotube itself as compared to other oxidizingagents. Experimental data and scanning electronic micrographs such asthose shown in FIGS. 4A and B reveal minimal or otherwise no measurableor visible loss of carbon mass from the ozone treated carbon nanotubeitself.

In addition to the surface and structural benefits on the carbonnanotube itself (i.e., more functional groups, less carbon destruction),a further benefit in using the preferred ozone treatment is the energyand cost savings imparted as a result of being able to carry out theozone oxidation treatment at a lower temperature range such as at roomtemperature. Thus, compared other oxidation processes, additionalheating equipment is not necessarily required.

Ozone Treated Carbon Nanotube Structures—General Summary

All of these benefits were also discovered to apply when treating anycarbon nanotube structure (e.g., fibril aggregates, mats, assemblages,three dimensional networks, rigid porous structures, etc.) with ozone.The ozone treated structures remained intact despite high degree ofsurface oxidation and all of the above recited weight gain, oxygencontent and acid titer characteristics apply as well.

Furthermore, as explained in the subsequent sections, similar benefitsare also conferred upon carbon nanotube structures which are made fromthese ozone oxidized carbon nanotubes.

Carbon nanotube structures include, but are not limited to the followinggroups: aggregates, assemblages, networks and rigid porous structures.

a. Aggregates are dense microscope particulate structures of entangledcarbon nanotubes and may resemble the morphology of birds nest, cottoncandy, combed yard or open net.

b. Assemblages are carbon nanotube structures which have relativelyuniform properties in along one, preferably two and most desirably threedimensional axis of the three dimensional assemblage. (E.g., U.S. Pat.No. 5,691,054 hereby incorporated by reference). Generally, assemblagesare formed by de-aggregating the carbon nanotube aggregate structure,and then reassembling them to form assemblages.

C. Networks are formed by linking individual functionalized carbonnanotubes together by using a linking molecule between thefunctionalized groups located on the surface of the carbon nanotubes.(E.g., PCT/US97/03553 or WO 97/32571, hereby incorporated by reference).

d. Rigid porous structures are formed by either linking the individualfunctionalized carbon nanotubes together without the use of a linkingmolecule, or by gluing carbon nanotube aggregate structures togetherwith a gluing agent. (E.g., U.S. Pat. No. 6,099,965, hereby incorporatedby reference).

In the preferred embodiment, ozone treated carbon nanotube structuresmay be made by forming the respective structure first and thensubjecting that structure to ozone treatment as described above in“Methods Of Treating Carbon Nanotubes With Ozone.” Alternatively, ozonetreated carbon nanotube structures may be formed from the ozone treatedcarbon nanotubes themselves.

Methods of Treating Aggregates of Carbon Nanotubes with Ozone

It has also been discovered that improved aggregates of carbon nanotubescan be formed by treatment with ozone in accordance with the preferredembodiment. As defined previously, aggregates are dense microscopicparticulate structures of entangled carbon nanotubes and can be madeusing any of the procedures previously described or incorporated byreference in the section entitled “Aggregates of Carbon Nanotubes andAssemblages.” Preferred aggregates have diameters less than 50 microns.

Consistent with the non-destructive treatment of carbon nanotubesdiscussed above, it has been further discovered that improved aggregatesof carbon nanotubes may be formed from the ozone treated carbonnanotubes described. Alternatively, improved aggregates of carbonnanotubes may formed by subjecting the untreated aggregates to ozonetreatment in accordance with the preferred embodiment.

When forming aggregates of carbon nanotubes from ozone treated carbonnanotubes, the individual carbon nanotubes are first subjected to ozonetreatment as disclosed in the previous section “Method Of TreatingCarbon Nanotubes With Ozone” to form ozone treated carbon nanotubes. Theozone treated carbon nanotubes are then formed into ozone treatedaggregates of carbon nanotubes using any method disclosed in thereferences incorporated by reference in the previous section entitled“Aggregates Of Carbon Nanotubes And Assemblages.”

Alternatively, if aggregates of untreated carbon nanotubes have alreadybeen formed, then those untreated aggregates may be subject to ozonetreatment in the same manner and conditions described in the previoussection “Method Of Treating Carbon Nanotubes With Ozone.”

In either embodiment, ozone treated aggregates of carbon nanotubes werediscovered to have a number of similar benefits as ozone treated carbonnanotubes. For example, the ozone treated aggregates exhibit anunexpected weight gain compared to untreated aggregates of carbonnanotubes. The increase in weight of the ozone treated aggregates ofcarbon nanotubes may be greater than 1%, and preferably greater than 5%,by comparison to the untreated aggregates of carbon nanotubes. In oneembodiment, the ozone treated carbon nanotube aggregates of thepreferred embodiment exhibit a 5 to 20% weight gain, more preferably a10 to 15% weight gain.

Moreover, it has also been unexpectedly found that the ozone treatmentin accordance with the preferred embodiment yields aggregates of carbonnanotubes with higher amounts of functional groups (especially acidicgroups) on the nanotube surface than treatments using other oxidizingagents. Specifically, ozone treated aggregates of carbon nanotubes canreach a higher acid titer (i.e., have more acidic groups) than thosetreated with other oxidizing agents such as nitric acid and hydrogenperoxide. Ozone treated aggregates of carbon nanotubes can reach an acidtiter between 1 meq/g to 2 meq/g.

Even more surprising is the discovery that the ozone treated aggregatesof carbon nanotubes substantially retain the structure of the originaluntreated aggregates while reaching acid titers between 1 meq/g to 2meq/g. This substantial retention of structure again confirms thenon-destructive effect of ozone when used in accordance with thepreferred embodiment. Aggregates of carbon nanotubes which have beensubjected to treatment with other oxidizing agents such as nitric acidare unable to reach such a high acid titer since those oxidizing agentswill engage in uncapping, stripping and shortening reactions on thecarbon nanotubes and thereby cause the aggregate structure to unraveland come apart.

Experiments further confirm that the use of ozone to treat aggregates ofcarbon nanotubes at the recited temperature range results in a moreefficient generation (higher percentage) of surface acidic groupscompared to carbon nanotubes which have been oxidized with nitric acidor hydrogen peroxide. Ozone treated aggregates of carbon nanotubes havehigher oxygen content than those oxidized with nitric acid or hydrogenperoxide. The surface of the ozone treated aggregates of carbonnanotubes can have a oxygen content greater than 4 percent (e.g.,4-10%), preferably greater than 6 percent (e.g., 6-10%). Experimentsalso showed that preferred functional groups resulting from the ozonetreatment were carboxyl, anhydride and ketone.

Methods of Treating Other Carbon Nanotube Structures with Ozone

Consistent with the previous section entitled “Methods Of TreatingAggregates Of Carbon Nanotubes With Ozone,” other improved carbonnanotube structures may be formed from the ozone treated carbonnanotubes. Alternatively, the untreated carbon nanotube structure may besubjected to ozone treatment in accordance with the preferredembodiment. Ozone treatment may be performed in gaseous or liquid phase.

Ozone treated carbon nanotubes can also be used to functionalize highquality extrudates which can be formed by using a small amount of watersoluble binder. In the preparation of extrudates, the functionalizedsurface of the nanotubes allows for improved binder dispersion duringthe mixing stage and minimizes the segregation of binder in thesubsequent heating step.

Methods for producing a network of carbon nanotubes comprising treatingcarbon nanotubes with ozone for a period of time sufficient tofunctionalize the surface of the carbon nanotubes, contacting the ozonetreated carbon nanotubes with a reactant suitable for adding a secondaryfunctional group to the surface of the carbon nanotube, and furthercontacting the secondarily treated nanotubes with a cross-linking agenteffective for producing a network of carbon nanotubes. A preferredcross-linking agent is a polyol, polyamine or polycarboxylic acid. Auseful polyol is a diol and a useful polyamine is a diamine.

In one aspect of the invention a network of carbon nanotubes is obtainedby first oxidizing the as-produced carbon nanotubes with ozone(alternatively, liquid phase ozone may be used), followed by subjectingthe ozone treated nanotubes to conditions which foster crosslinking. Forexample, heating the ozone treated nanotubes in a temperature range from180° C. to 650° C. results in crosslinking the ozone treated nanotubestogether with elimination of the oxygen containing moieties of the ozonetreated nanotubes.

The invention also includes three-dimensional networks formed by linkingthe ozone treated nanotubes of the preferred embodiment. These complexesinclude at least two surface-modified nanotubes linked by one or morelinkers comprising a direct bond or chemical moiety. These networkscomprise porous media of remarkably uniform equivalent pore size. Theyare useful as adsorbents, catalyst supports and separation media.

Stable, porous 3-dimensional networks or structures with meso- andmacropores (pores>2 nm) are very useful as catalysts or chromatographysupports. Since nanotubes can be dispersed on an individualized basis, awell-dispersed sample which is stabilized by cross-links allows one toconstruct such a support. Ozone treated nanotubes are ideal for thisapplication since they are easily dispersed in aqueous or polar mediaand the oxygen-containing moieties present on the oxidized nanotubesprovide cross-link points. Additionally, the oxygen containing moietiesalso provide points to support the catalytic or chromatographic sites.The end result is a rigid, 3-dimensional structure with its totalsurface area accessible with secondary group sites on which to supportthe active agent.

Although the interstices between these nanotubes are irregular in bothsize and shape, they can be thought of as pores and characterized by themethods used to characterize porous media. The size of the intersticesin such networks can be controlled by the concentration and level ofdispersion of nanotubes, and the concentration and chain lengths of thecross-linking agents. Such materials can act as structured catalystsupports and may be tailored to exclude or include molecules of acertain size. Aside from conventional industrial catalysis, they havespecial applications as large pore supports for biocatalysts.

Typical applications for these supports in catalysis include their useas a highly porous support for metal catalysts laid down byimpregnation, e.g., precious metal hydrogenation catalysts. Moreover,the ability to anchor molecular catalysts by tether to the support viathe secondary groups combined with the very high porosity of thestructure allows one to carry out homogeneous reactions in aheterogeneous manner. The tethered molecular catalyst is essentiallydangling in a continuous liquid phase, similar to a homogeneous reactor,in which it can make use of the advantages in selectivities and ratesthat go along with homogeneous reactions. However, being tethered to thesolid support allows easy separation and recovery of the active, and inmany cases, very expensive catalyst.

These stable, rigid structures also permits carrying out heretofore verydifficult reactions, such as asymmetric syntheses or affinitychromatography by attaching a suitable enantiomeric catalyst orselective substrate to the support. The rigid networks can also serve asthe backbone in biomimetic systems for molecular recognition. Suchsystems have been described in U.S. Pat. No. 5,110,833 and InternationalPatent Publication No. WO93/19844. The appropriate choices forcross-linkers and complexing agents allow for stabilization of specificmolecular frameworks.

Methods of Preparing Ozone Treated Rigid Porous Structures

Rigid porous structures are prepared by first preparing ozone treatednanotubes as described above, dispersing them in a medium to form asuspension, separating the medium from the suspension to form a porousstructure, wherein the ozone treated nanotubes are furtherinterconnected to form a rigid porous structure, all in accordance withmethods more particularly described in U.S. Pat. No. 6,099,965 entitled“Rigid Porous Carbon Structures, Methods of Making, Methods of Using andProducts Containing Same” filed on May 15, 1997, hereby incorporated byreference.

The hard, high porosity structures can be formed from carbon nanotubesor nanotube aggregates. In order to increase the stability of thenanotube structures, it is also possible to deposit polymer at theintersections of the structure. This may be achieved by infiltrating theassemblage with a dilute solution of low molecular weight polymer cement(i.e., less than about 1,000 MW) and allowing the solvent to evaporate.Capillary forces will concentrate the polymer at nanotube intersections.It is understood that in order to substantially improve the stiffnessand integrity of the structure, only a small fraction of the nanotubeintersections need be cemented.

The nanotubes may be uniformly and evenly distributed throughout thestructure or in the form of aggregate particles interconnected to formthe structure. When the former is desired, the nanotubes are dispersedthoroughly in the medium to form a dispersion of individual nanotubes.When the latter is desired, nanotube aggregates are dispersed in themedium to form a slurry and said aggregate particles are connectedtogether with a gluing agent to form said structure.

The medium used may be selected from the group consisting of water andorganic solvents. Preferably, the medium comprises a dispersant selectedfrom the group consisting of alcohols, glycerin, surfactants,polyethylene glycol, polyethylene imines and polypropylene glycol.

The medium should be selected which: (1) allows for fine dispersion ofthe gluing agent in the aggregates; and (2) also acts as a templatingagent to keep the internal structure of the aggregates from collapsingas the mix dries down.

One preferred embodiment employs a combination of polyethylene glycol(PEG) and glycerol dissolved in water or alcohol as the dispersingmedium, and a carbonizable material such as low MW phenol-formaldehyderesins or other carbonizable polymers or carbohydrates (starch orsugar). Once the rigid porous structure has been prepared, it can thenbe treated with ozone in accordance with the preferred embodiment inpreparation for use in electrochemical capacitors, for example.

In other words, a preferred embodiment includes a method for forming acatalyst support comprising the steps of: forming a rigid porousstructure comprising carbon nanotubes, and contacting said rigid porousstructure with ozone at a temperature range between 0° C. to 100° C.under conditions sufficient to form a functionalized rigid porousstructure which is greater in weight than said rigid porous structure.

In another embodiment, if ozone treated carbon nanotubes or aggregatesare employed to form the rigid porous structure, the nanotubes aretreated with ozone in accordance with the preferred embodiment prior todispersing in the medium and are self-adhering forming the rigidstructure by binding at the nanotube intersections. The structure may besubsequently pyrolized to remove oxygen. A useful pyrolysis temperaturerange is from about 200° C. to about 2000° C. and preferably from about200° C. to about 900° C.

According to another embodiment, the nanotubes are dispersed in saidsuspension with gluing agents and the gluing agents bond said nanotubesto form said rigid structure. Preferably, the gluing agent comprisescarbon, even more preferably the gluing agent is selected from amaterial that, when pyrolized, leaves only carbon. Accordingly, thestructure formed with such a gluing may be subsequently pyrolized toconvert the gluing agent to carbon. Additional methods and details forforming rigid porous structures are found in U.S. Pat. No. 6,099,965,hereby incorporated by reference.

Preferably, the gluing agents are selected from the group consisting ofacrylic polymers, carboxylic polymers, cellulose, carbohydrates,polyethylene, polystyrene, nylon, polyurethane, polyester, polyamides,polyvinyl acetate/alcohol emulsions or resins, amino resins, epoxyresins and phenolic resins.

According to further embodiments of the invention, the step ofseparating comprises filtering the suspension or evaporating the mediumfrom said suspension.

According to yet another embodiment, the suspension is a gel or pastecomprising the nanotubes in a fluid and the separating comprises thesteps of:

-   (a) heating the gel or paste in a pressure vessel to a temperature    above the critical temperature of the fluid;-   (b) removing supercritical fluid from the pressure vessel; and-   (c) removing the structure from the pressure vessel.

Isotropic slurry dispersions of nanotube aggregates insolvent/dispersant mixtures containing gluing agent can be accomplishedusing a Waring blender or a kneader without disrupting the aggregates.The nanotube aggregates trap the resin particles and keep themdistributed.

These mixtures can be used as is, or can be filtered to removesufficient solvent to obtain cakes with high nanotube contents (5-20%dry weight basis). The cake can be molded, extruded or pelletized. Themolded shapes are sufficiently stable so that further drying occurswithout collapse of the form. On removing solvent, disperant molecules,along with particles of gluing agent are concentrated and will collectat nanotube crossing points both within the nanotube aggregates, and atthe outer edges of the aggregates. As the mixture is further dried downand eventually carbonized, nanotube strands within the aggregates andthe aggregates themselves are glued together at contact points. Sincethe aggregate structures do not collapse, a relatively hard, veryporous, low density particle is formed. The resulting rigid porousstructure can then be subjected to ozone treatment under the same mannerand conditions described in the “Method Of Treating Carbon NanotubesWith Ozone.”

As set forth above, the rigid, porous structures may also be formedusing ozone treated nanotubes with or without a gluing agent. Carbonnanotubes become self-adhering after being oxidized by ozone. Very hard,dense mats are foamed by highly dispersing the oxidized nanotubes (asindividualized strands), filtering and drying. The dried mats havedensities between 1-1.2 g/cc, depending on oxygen content, and are hardenough to be ground and sized by sieving. Measured surface areas areabout 275 m²/g.

The ozone treated nanotubes may also be used in conjunction with agluing agent. Ozone treated nanotubes are good starting materials sincethey have attachment points to stick both gluing agents and templatingagents. The latter serve to retain the internal structure of theparticles or mats as they dry, thus preserving the high porosity and lowdensity of the original nanotube aggregates. Good dispersions areobtained by slurrying ozone treated nanotubes with materials such aspolyethyleneimine cellulose (PEI Cell), where the basic imine functionsform strong electrostatic interactions with carboxylic acidfunctionalized fibrils. The mix is filtered to form mats. Pyrolizing themats at temperatures greater than 650° C. in an inert atmosphereconverts the PEI Cell to carbon which acts to fuse the nanotubeaggregates together into hard structures. The result is a rigid,substantially pure carbon structure, which can then be further treatedagain with ozone if desired.

Solid ingredients can also be incorporated within the structure bymixing the additives with the nanotube dispersion prior to formation ofthe structure. The content of other solids in the dry structure may bemade as high as fifty parts solids per part of nanotubes.

According to one preferred embodiment, nanotubes are dispersed at highshear in a highshear mixer, e.g., a Waring Blender. The dispersion maycontain broadly from 0.01 to 10% nanotubes in water, ethanol, mineralspirits, etc. This procedure adequately opens nanotube bundles, i.e.tightly wound bundles of nanotubes, and disperses the nanotubes to formself-supporting mats after filtration and drying. The application ofhigh shear mixing may take up to several hours. Mats prepared by thismethod, however, are not free of aggregates.

If the high shear procedure is followed by ultrasonication, dispersionis improved. Dilution to 0.1% or less aids ultrasonication. Thus, 200 ccof 0.1% fibrils, for example, may be sonified by a Bronson SonifierProbe (450 watt power supply) for 5 minutes or more to further improvethe dispersion.

To achieve the highest degrees of dispersion, i.e., a dispersion whichis free or virtually free of nanotube aggregates, sonication must takeplace either at very low concentration in a compatible liquid, e.g., at0.001% to 0.01% concentration in ethanol or at higher concentratione.g., 0.1% in water to which a surfactant, e.g., Triton X-100, has beenadded in a concentration of about 0.5%. The mat which is subsequentlyformed may be rinsed free or substantially free of surfactant bysequential additions of water followed by vacuum filtration. The matthus formed can then be treated with ozone in accordance with thepreferred embodiment.

Particulate solids such as MnO₂ (for batteries) and Al₂O₃ (for hightemperature gaskets) may be added to the ozone treated nanotubedispersion prior to mat formation at up to 50 parts added solids perpart of nanotubes.

Reinforcing webs and scrims may be incorporated on or in the mats duringformation. Examples are polypropylene mesh and expanded nickel screen.

Catalyst Supports

Carbon nanotube structures such as carbon nanotube aggregates, threedimensional networks or rigid porous structures can be used as catalystsupports, including but not limited to catalyst supports for catalystswhich catalyze the formation of carbon nanotubes. Of these choices,rigid porous structures offer the preferred combination of size,strength and surface area as catalyst supports.

Thus, the preferred embodiments include forming catalyst supports byfunctionalizing carbon nanotube structures such as aggregates, rigidporous structures or three dimensional networks via treatment withozone. Aggregates may be prepared by any methods discussed previously,including those disclosed in U.S. Pat. No. 5,165,909 to Tennent; U.S.Pat. No. 5,456,897 to Moy et al.; Snyder et al., U.S. Pat. No.5,707,916, filed May 1, 1991, and PCT Application No. US89/00322, filedJan. 28, 1989 (“Carbon Fibrils”) WO 89/07163, and Moy et al., U.S. Pat.No. 5,456,897 filed Aug. 2, 1994 and PCT Application No. US90/05498,filed Sep. 27, 1990 (“Battery”) WO 91/05089, and U.S. Pat. No. 5,500,200to Mandeville et al., filed Jun. 7, 1995 and U.S. Pat. No. 5,456,897filed Aug. 2, 1994 and U.S. Pat. No. 5,569,635 filed Oct. 11, 1994 byMoy et al, all of which are hereby incorporated by reference.

Rigid porous structures may be made using any methods discussedpreviously, including those disclosed in U.S. Pat. No. 6.432,866 toTennent et al., hereby incorporated by reference. In summary, rigidporous structures such as those described above or in U.S. Pat. No.6,432,866, are typically prepared by oxidizing the nanotubes and thenheating them to cause cross-linking between the nanotubes or areprepared by mixing nanotubes with a gluing agent and heating tocarbonize the gluing agent. Three dimensional networks may be made usingany methods disclosed in U.S. Pat. No. 5,968,650 to Tennent et al.,hereby incorporated by reference.

One important aspect of a catalyst support is that the support must beable to hold onto the catalyst during the course of the catalyticreaction, whether via chemical bonding, adhesion, or any other forceswhich permit the catalyst to remain on the support itself. To promotestable and sufficient bonding between the catalyst and the support, itis preferred that the support itself contain a number of functionalgroups on its surface which the catalyst will bind to or react with soas to establish the desired bonding between the catalyst and thesupport.

Another important aspect of a catalyst support is that the support beable to maintain its structure during the course of a reaction insteadof deteriorating or breaking apart.

In the preferred embodiment, catalyst supports are created by treatingor contacting the carbon nanotube structure such as carbon nanotubeaggregates, three dimensional networks or rigid porous structures withozone at a temperature range between 0° C. to 100° C., preferablybetween 0° C. to 60° C., most preferably between 20° C. to 50° C. or atabout room temperature. Ozone is especially preferred since it has beendiscovered to add functional groups onto the carbon nanotube structureswithout weakening or destroying the crosslinking, glue or other forcesor bonds which hold those structures together (i.e., no observablechange in structure).

Other strong oxiding agents such as nitric acid can also be used to addfunctional groups onto the surface of these structures, however, it hasbeen discovered that nitric acid is more likely to hydrolyze crosslinkedbonds or dissolve the glue bonds or diminish other bonds so as to eitherweaken the integrity of the original support structure. Conversely, weakoxidizing agents such as hydrogen peroxide, while the) may not weakenthe integrity of the support structure, do not result in enoughfunctional groups being added to the support surface (i.e., low acidtiters) to be of commercially practical use.

Thus, to obtain catalyst support structures with sufficient structuralintegrity and sufficient functional groups, it is preferred to preparecatalyst support structures by contacting the carbon nanotube structuressuch as carbon nanotube aggregates, three dimensional networks or rigidporous structures with ozone as the preferred oxidizing agent, sincethat will result in catalyst supports with higher concentrationfunctional groups (i.e., higher acid titers, and thus better retentionof the catalyst) and stronger structural integrity as compared totreating those structures with other oxidizing agents.

Preferred catalyst supports comprise ozone treated carbon nanotubestructures which exhibit upon titration an acid titer grater than 1meq/g such as from 1 to 2 meq/g. These ozone treated carbon nanotubestructures further exhibit no observable change in structure incomparison to the original untreated carbon nanotube structures. Theozone treated carbon nanotube structure can also exhibit a weight gainof greater than 1%, preferably 5 to 20% weight gain, more preferably a10 to 15% weight gain. Other acid titer, oxygen contents, and weightgain characteristics as described earlier apply to the functionalizedstructures used as catalyst supports as well.

Electrochemical Capacitors

Carbon nanotubes are electrically conductive. Electrodes and their usein electrochemical capacitors comprising carbon nanotubes and/orfunctionalized carbon nanotubes have been described in U.S. Pat. No.6,031.711 entitled “Graphitic Nanofibers in Electrochemical Capacitors,”filed on May 15, 1997 and is incorporated herein by reference.

Further details about electrochemical capacitors based on catalyticallygrown carbon nanotubes are disclosed in Chumming Niu et al., “High PowerElectrochemical Capacitors based on Carbon Nanotube Electrodes,” inApplied Physics Letters 70(11), pp. 1480-1482, Mar. 17, 1997incorporated herein by reference.

The quality of sheet electrode depends on the microstructure of theelectrode, the density of the electrode, the functionality of theelectrode surface and mechanical integrity of the electrode structure.

The microstructures of the electrode, namely, pore size and sizedistribution determines the ionic resistance of electrolyte in theelectrode. The surface area residing in micropores (pore diameter<2 nm)is considered inaccessible for the formation of a double layer (2). Onthe other hand, distributed pore sizes, multiple-pore geometries (deadend pores, slit pores, cylindrical pores, etc.) and surface propertiesusually give rise to a distributed time constant. The energy stored inan electrode with a distributed time constant can be accessed only withdifferent rates. The rapid discharge needed for pulsed power is notfeasible with such an electrode.

The density of the electrode determines its volumetric capacitance. Anelectrode with density less than 0.4 g/cc is not practical for realdevices. Simply, the low-density electrode will take up too muchelectrolyte, which will decrease both volumetric and gravimetriccapacitance of the device.

The surface of the carbon nanotubes is related to the wetting propertiesof electrodes towards electrolytes. The surface of as-produced,catalytically grown carbon nanotubes is hydrophobic. The hydrophobicsurface properties of the as produced carbon nanotubes can be changed tohydrophilic by treatment of the as produced carbon nanotubes oraggregates of carbon nanotubes with ozone in accordance with thepreferred embodiment. Furthermore, the capacitance can be increased byfurther attaching redox groups on the surface of the carbon nanotubes.

Finally, the structural integrity of the electrodes is critical toreproducibility and long term stability of the device. Mechanicalstrength of electrodes incorporating carbon nanotubes is determined bythe degree of entanglement of the carbon nanotube and bonding betweencarbon nanotubes in the electrode. A high degree of entanglement andcarbon nanotube bonding can also improve the conductivity, which iscritical to the power performance of an electrode. The specificcapacitance (D.C. capacitance) of the electrodes made from gas-phasetreated fibrils was about 40 F/g.

One aspect of the present invention relates to preparing electrodes andelectrochemical capacitors from ozone treated carbon nanotubes. Broadly,as prepared carbon nanotubes have been treated with ozone in accordancewith the preferred embodiment to provide surface oxidized, single walledor multiwalled carbon nanotubes which can be used to prepare theelectrodes of the invention.

In another aspect of the invention, the ozone treated nanotubes can befurther treated with a reactant suitable to react with moieties presenton the oxidized nanotubes to form nanotubes having secondary groups onits surface which are also useful in preparing the electrodes of thepresent invention.

Electrodes are assembled by simple filtration of slurries of the ozonetreated nanotubes. Thickness is controlled by the quantity of materialused and the geometry, assuming the density has been anticipated basedon experience. It may be necessary to adjust thickness to getselfsupporting felts.

The electrodes are advantageously characterized by cyclic voltammetry,conductivity and DC capacitance measurement.

EXAMPLES

The following examples serve to provide further appreciation of theinvention but are not meant in any way to restrict the effective scopeof the invention.

Method of Measuring Titer

As an initial matter, measuring an acid titer can be effected in anumber of ways. In one embodiment, 0.10 g of fibrils are transferred toa Waring Laboratory Blender containing 350-400 CC of D.I. water. Thefibrils are blended at slow speed for 10-15 minutes until all fibrils inthe water phase appeared to be homogenously black. 10 CC of standard0.10 N sodium hydroxide solution is added to the blender containing finefibrils slurry and again blended for 4-5 minutes. The slurry is thentransferred to a beaker containing a stirring rod and a pH electrode. Asolution of hydrochloric acid of known strength is then added graduallyto neutralize the fibrils slurry at pH 7.0. The volume of hydrochloricacid used to neutralize fibrils slurry is noted and used in calculationof meq/g of fibrils. The results of titer measurement may be expressedin either milliequivalence per nanotube weight (i.e., meq/g) ormilliequivalence per nanotube surface area (i.e., meq/m²).

Example 1

Ozone was generated via an air purifier made by Del Industry, San LuisObispo, Calif., which can generate ozone at a rate of 250 mg/hr. Amixture of ozone and air (0.29% ozone) at a flow rate of 1200 mL/min wasthen passed though a 1-inch (OD) reactor tube packed with dry as-madefibrils. The weight of fibrils before and after ozone treatment wererecorded. The reaction was allowed to proceed for a period of 3 to 45hours at room temperature.

In a separate experiment, 20 grams of as-made fibrils were placed in aflask containing 500 mL 30% or 60% nitric acid. The reaction flask wasthen heated to reflux temperature of 95-120° C. for 4-6 hours. After thereaction was stopped, the fibrils were cooled to room temperature,filtered, and washed with water until neutral. In another separateexperiment, 20 grams of as-made fibrils were placed in a flaskcontaining 376.2 grams of 30% H₂O₂ in molar ratio of 1:2. Thetemperature was set to be in the vicinity of 35° C., but it rose toreflux temperature quickly. A water bath was applied to maintain thereaction temperature at 30° C. After 2-hour reaction, the slurry wasfiltered and washed until all the residual H₂O₂ was removed.

The results are reported below:

TABLE 2 Weight changes during fibril oxidation Sample Weight Run TimeWeight Change No. Oxidant (g) (hr) (%) 1 O₃/air 3 20 +13.9  2 O₃/air 320 +8.0 3 O₃/air 10 45 +10.4  4 O₃/air 9 45 +10.2  5 O₃/air 3 25 +12.1 6 30% HNO₃ 20 6 −6.4 7 60% HNO₃ 20 4 −15.3  8 30% H₂O₂ 20 2 −1.7

Experimental data confirmed that oxidation of carbon nanotubes usingozone at room temperature as compared to other oxidizing agents such asnitric acid or hydrogen peroxide resulted in a surprising difference inweight of the final product. Namely, carbon nanotubes subjected to ozonetreatment at room temperature posted a significant weight gain insteadof weight loss as compared to carbon nanotubes which have been subjectedto other oxidation treatment. This is surprising given that ozone is astrong oxidant.

With respect to using nitric acid as the oxidizing agent, the nitricacid strength and reaction condition affected the degree of weight lossThe weight loss is attributed to the oxidation of carbon to form CO orCO₂, which evolute from the reactor system.

Example 2

3 grams of dry fibrils was placed in a vertical reactor andozone-containing air was passed through them at room temperature. Thereactor was shut off periodically at every hour and the total weight offibrils plus the reactor tube were measured on an electric balance. Theweight gain of fibrils against reaction time was then obtained aftertarring off the reactor weight.

The results of this measurement are displayed in FIG. 7, which showsthat the sample's weight increased over the course of reaction andleveled off after approximately 15 hours of reaction lime.

Example 3

Various oxidized fibrils prepared according to the methods in Example 1such as Samples 1-3 and 5-7 were measured to determine their relativeamount of acidic groups through titration. 0.25 gram of each sample wasplaced into a flask containing 300 mL D.I. water and the slurry wastitrated with 0.1N NaOH. The consumption of NaOH was translated into thequantity of total surface acidic groups as meq/g.

TABLE 3 Measurement of surface acidic groups through titration. Sampleweight Weight change Titer Sample (g) Oxidant (%) (meq/g) 1 3 O₃/air13.95 1.45 2 3 O₃/air 8.0 1.07 3 10 O₃/air 10.40 1.57 5 3 O₃/air 12.121.16 8 20 30% HNO₃ −15.28  1.20 6 20 60% HNO₃ −6.35 0.58 7 20 30% H₂O₂−4.0  0.128

As shown in Table 3, more acidic groups can be deposited onto thesurface of carbon nanotubes when oxidized with ozone at room temperaturethan with other oxidizing agents such as nitric acid or hydrogenperoxide. In other words, treatment of carbon nanotubes with ozone atroom temperature will yield nanotubes with a higher titer (i.e., anindicator of acidic groups) than nanotubes resulting from treatment withother oxidizing agents.

Additionally, even though carbon nanotubes treated with nitric acid wasable to reach a titer within the range of the lowest titer yielded byozone treatment, such nitric acid treated nanotubes may be unsuitablefor certain applications due to the significant weight loss thataccompany such titer range.

Table 3 further confirms that ozone treatment at room temperature yieldscarbon nanotubes which are significantly more acidic than those by weakacids such as hydrogen peroxide or diluted nitric acid.

Example 4

To compare the degree of damage done to the carbon nanotube surface bythe oxidation treatments, the surface area of the treated fibrils wasmeasured using an Autosorb-1 instrument. For the room temperature ozoneoxidized fibrils, the average BET surface area at −196° C. were all inthe range of 240-250 m²/g, which is very close to as-made untreatedfibrils. On the other hand, the average BET surface area (measured at196° C.) of oxidized fibrils treated with nitric acid increased from 250m²/g to 341 m²/g.

60% Nitric As-made acid treated (surface Ozone treated (surface Titer(meq/m² Sample area) (surface area) area) surface area) 9 240 m²/g 247m²/g 0.0055 10 240 m²/g 323 m²/g 0.0029 11 240 m²/g 404 m²/g 0.0028

Such increase in surface area is believed to be caused by the strippingof layers or uncapping of nanotubes. Therefore, minimal or no measurabledamage was generated on the surface of carbon nanotubes during ozonetreatment at room temperature.

Example 5

XPS (X-ray Photoelectron Spectroscopy) was applied to characterize thefibril surface of fibril aggregates treated using nitric acid, hydrogenperoxide, gaseous ozone, (all as described earlier) and liquid ozone (asdescribed later) as compared to plain untreated fibril aggregates.

Three sets of characteristics were measured: the atomic concentration ofsurface oxygen; carbon functionality and oxygen functionality.

Table 4A and B Surface Composition and Carbon Functionality of DifferentFibrils

TABLE 4A Fibril Sample C (%) O (%) Plain CC 98.0 2.0 Nitric acid CC 96.43.6 H₂O₂ CC 96.3 3.7 O₃ (gas phase) CC 93.1 6.9 O₃ (liquid phase) CC95.8 4.2

TABLE 4B Graphitic —C—OH —C═O —CO₂H —CO₃ Sample C (%) (%) (%) (%) (%)Plain CC 80.0 11.4 3.4 2.3 2.9 Nitric Acid CC 73.2 12.7 6.0 3.4 4.6 H₂O₂CC 77.6 12.4 3.4 3.0 3.6 O₃ (gas) CC 71.8 14.3 6.3 4.0 3.6 O₃ (liquid)CC 74.7 12.2 5.5 3.1 4.4

As expected, plain fibril had the least amount of oxygen on the surface,which is generally created upon exposure to air after production. Table4A further confirms that oxidation yields more oxygenated groups ontothe fibril surface as the oxygen content was increased for all fibrilstreated with oxidizing agents. Table 4A and 4B both confirm that thehighest oxygen content was exhibited in gaseous ozone treated carbonnanotubes at room temperature.

Table 4B further displays the breakdown of those oxygenated contents onthe fibril surfaces and confirms that different oxidizing agents yieldsdifferent quantities of different functional groups. For example, carbonnanotubes treated with H₂O₂ treatment created more —OH groups whilenitric acid generated more —COOH as a percentage increase of the totalfunctional groups generated. The fibrils treated with gaseous ozone atroom temperature yielded the greatest —COOH increase.

Further analysis on the oxygen content so as to determine moreinformation on the surface functional groups of the fibrils are shown inTable 4C and FIG. 8. FIG. 8 showed the oxygen spectra at the 1 s region.The raw data are represented by the plotted dots while the fitted dataand deconvoluted signals are represented by the solid lines. The peaksat 531 eV, 533 eV and 535 eV are assigned to the C—O, C═O and H₂Oconfiguration. The data in FIG. 8 was summed up in Table 4C below:

TABLE 4C Oxygen functionality analysis Sample O—(C) (%) O═(C) (%) H₂O(%) Plain CC 77.2 16.8 6.0 CC—H₂O₂ 66.5 25.0 8.6 CC-nitric acid 38.551.3 10.2 CC—O₃ (Gas phase) 37.3 59.1 3.6

As shown in Table 4C, the oxygen groups of plain fibril generally are ina single bond “O—” conformation. Fibrils subjected to H₂O₂ oxidizationyielded more oxygen groups in the form of —OH and C═O. However, sincethe single bond —O confirmation still dominated, this may explain thelower titer results of nanotubes treated with hydrogen peroxide.

On the other hand, nitric acid or ozone oxidation yielded carbonnanotubes in which the surface oxygen group was switched from beingpredominately single bond —O conformation to predominantly double bond═O conformation. Furthermore, fibrils subjected to gas phase ozonetreatment resulted in less moisture content as well.

Example 6

Ozone treated fibrils at room temperature were examined using SEM.Analysis of the fibril aggregate structure under the scanning electronmicroscope as shown in FIG. 9 confirmed that the fibril aggregatesstructure remained intact after the ozone treatment at room temperature.

Example 7

Differences between untreated fibril aggregates, fibril aggregatesoxidized with nitric acid and fibril aggregates oxidized with ozone weredemonstrated by the following test. Three sets of fibril aggregates wereprepared in the same manner. The first fibril aggregates were leftuntreated after preparation. The second fibril aggregates weresubsequently treated with nitric acid at room temperature. The thirdfibril aggregates were subsequently treated with ozone at roomtemperature. The three sets of fibril aggregates were dispersed in waterand the following differences were observed:

Fibril Hydrophobic Wetting Structural Aggregate vs. HydrophillicCharacteristic Retention Untreated hydrophobic non-wetting retainsstructure Oxidized with hydrophillic wetting breaks apart Nitric AcidOxidized with hydrophillic wetting retains structure OzoneThus, fibril aggregates treated with ozone have differentcharacteristics from untreated fibril aggregates (i.e., hydrophobic andnon wetting v. hydrophillic and wetting) as well as from fibrilaggregates treated with strong oxidants such as nitric acid (i.e.,retains structure v. breaks apart).

Example 8

The following experiments were performed using the procedure describedin Example 1, except a heating device such as heating tape was appliedto the 1-inch reactor tube. Ozone treatment of dry fibril aggreagateswas carried out at room and slightly elevated temperatures. Thefollowing data was obtained.

Sample Temperature Reaction Weight Titer weight (g) (° C.) time (h)change (%) (meq/g) 1 25 1 0.52 0.3 1 50 1 1.38 0.5 1 50 3 2.55 0.9 1 1001 0   0.1 1 100 8 -4.9  0.2

These experiment confirm that conditions such as temperature andreaction time can affect the resulting ozone treatment on the carbonnanotubes. Reactions carried out at temperatures between 0° C. to 100°C. result in the efficient generation of abundant surface functionalgroups without the resultant destruction of fibril aggregate mass.Experiments carried out at 100° C. or greater for extended periods oftime can lead to more substantial reactions between ozone with carbon toform CO and CO₂ resulting in loss of fibril aggregate mass.

Example 9

1 gram of nitric acid treated fibrils exhibiting a titer of 0.93 meq/gwas subject to ozone treatment at room temperature for a period of fivehours. The treatment followed the same procedure as described inExample 1. The resulting materials exhibited a weight gain of 10.6% andits titer was improved to 1.84 meq/g. This titer level is unexpected andsurprising since it could not be achieved with the nitric acid oxidationwithout substantially damaging and disassembling the carbon nanotubestructures.

Example 10

An impure mixture of single wall carbon nanotubes and amorphous carbonwas subjected to selective oxidation treatment with nitric acid toreduce soot and amorphous carbon. The resulting oxidized mixture asanalyzed by electron micrograph was approximately 20-50% impure andexhibited upon titration an acid tier of 0.9 meq/g. Using the proceduredescribed in Example 1, the oxidized mixture was subjected to ozonetreatment at room temperature for approximately 5 hours. The ozonetreated mixture exhibited upon titration an acid titer of 2.86 meq/g.

Example 11

The ozone treated fibrils were further blended with water to form aslurry with solid content of 15˜25% and then subject to extrusionthrough a ⅛ inch die. Cylindrical extrudates of oxidized fibrils wereobtained and dried at 180° C. for 12 hours. The extrudates appeared tobe very strong similar to those generated with nitric acid oxidizedfibrils. It is believed that the abundant surface functional groups maycross-link to self-assemble a rigid porous structure. As-made fibrilscannot form this type of structures without proper binding materials.

Example 12

20 grams of as-made CC nanotubes were placed in a Waring blender. 60grams gluing agent containing 10% polymer, e.g. polyacrylic acid(MW=15000), in aqueous solution, was added into the carbon nanotubes andthe mixture was blended till uniform. The resulting material was thenpush through a ⅛″ tie via a Brabender extruder followed by drying andcalcination at temperatures between 300 and 600° C. in argon. Theresulted extrudates were then treated with ozone for 20 hours using theprocedure described in Example 1. After ozone treatment, the extrudatesexhibited an acid titer of 0.79˜0.93.

Example 13

10 grams of as-made fibrils were placed in cyclohexane in a reactionflask. Ozone containing gas was then passed through the slurry withvigorous agitation. The reaction was allowed to proceed for 8-10 hoursunder ambient conditions. After reaction, fibrils was filtered, washedand dried. Suitable liquid media could, but not limited to be, water,saturated alcohol, saturated hydrocarbon or cyclic hydrocarbons, etc.The reaction can also be carried out at elevated temperatures andpressures up to supercritical conditions.

Example 14

1/16 inch carbon nanotube extrudates were placed into rotating drumreactors. Ozone containing gas generated from the feed of pure oxygengas into an ozone generator was introduced via a sparger into therotating drum reactors at rates of 1 L/min and 2.5 L/min. Two sampleswere titrated from each reactor at selected times to compare thefunctionalization uniformity. The following results were obtained:

Run Time Ozone Flow Rate Titer−Sample 1 Titer−Sample 2 (hr) (L/min)(meq/g) (meq/g) 3 1 0.23 0.19 8 1 0.29 0.26 24 1 0.44 0.49 48 1 0.690.63

Run Time Ozone Flow Rate Titer-Sample 1 Titer-Sample 2 (hr) (L/min)(meq/g) (meq/g) 3 2.5 0.08 0.18 8 2.5 0.21 0.29 24 2.5 0.40 0.41

The terms and expressions which have been employed are used as terms ofdescription and not of limitations, and there is no intention in the useof such terms or expressions of excluding any equivalents of thefeatures shown and described as portions thereof, it being recognizedthat various modifications are possible within the scope of theinvention.

1. A method of functionalizing carbon nanotubes comprising the step of:contacting carbon nanotubes with ozone at a temperature range between 0°C. to 100° C. under conditions sufficient to form functionalizednanotubes which are greater in weight than said carbon nanotubes.
 2. Themethod of claim 1, wherein said temperature range is between 0° C. to60° C.
 3. The method of claim 1, wherein said temperature range isbetween 20° C. to 50° C.
 4. The method of claim 1, wherein said carbonnanotubes are multiwalled carbon nanotubes having a diameter of lessthan 0.1 micron.
 5. The method of claim 1, wherein said carbon nanotubesare single walled nanotubes having a diameter less than 5 nanometer. 6.The method of claim 1, wherein the surface of said functionalizednanotubes have an oxygen content greater than 4 percent.
 7. The methodof claim 1, wherein the surface of said functionalized nanotubes have anoxygen content greater than 6 percent.
 8. The method of claim 1, whereinsaid functionalized nanotubes exhibit upon titration an acid titergreater than 2 meq/g.
 9. The method of claim 1, wherein saidfunctionalized nanotubes exhibit upon titration an acid titer from 1.6to 2.2 meq/g.
 10. The method of claim 1, wherein said functionalizednanotubes exhibit upon titration an acid titer from 2.5 to 3.5 meq/g.11. The method of claim 1, wherein said functionalized nanotubes exhibitupon titration an acid titer which is at least 1.5 meq/g greater thanthe acid titer of said carbon nanotubes.
 12. The method of claim 1,wherein said functionalized nanotubes exhibit upon titration an acidtiter which is at least 2 meq/g greater than the acid titer of saidcarbon nanotubes.
 13. The method of claim 1, wherein said functionalizednanotubes exhibit upon titration an acid titer which is 1.5 meq/g to 3meq/g greater than said carbon nanotubes.
 14. The method of claim 1,wherein said functionalized carbon nanotubes exhibit a weight gaingreater than 5% by comparison to said carbon nanotubes.
 15. The methodof claim 1, wherein said functionalized carbon nanotubes exhibit aweight gain from 5% to 20% by comparison with said carbon nanotube. 16.The method of claim 1, wherein said functionalized carbon nanotubesexhibit a weight gain from 10% to 15% by comparison with said saidcarbon nanotube.
 17. The method of claim 1, further comprising treatingsaid functionalized carbon nanotubes with a reactant suitable to reactwith moieties of said functionalized carbon nanotubes thereby adding atleast a secondary group onto the surface of said functionalizednanotubes.
 18. The method of claim 17, wherein said additional secondarygroup is selected from the group consisting of an alkyl or aryl silanewherein said alkyl has C₁ to C₁₈, said aryl has C₁ to C₁₈, an alkyl ofC₁ to C₁₈ or an aralkyl group of C₁ to C₁₈, a hydroxyl group of C₁ toC₁₈ and an amine group of C₁ to C₁₈.
 19. The method of claim 17, whereinsaid additional secondary group is a fluorocarbon.
 20. The method ofclaim 1 further comprising dispersing said functionalized carbonnanotubes into a liquid medium to form a mixture; filtering said mediumto collect a residue of functionalized carbon nanotubes; and drying saidresidue to form a mat.
 21. The method of claim 20, further comprisingheating said mat to a temperature range of 200° C. to 900° C.
 22. Themethod of claim 20, further comprising forming said mat into anelectrode.
 23. The method of claim 1, wherein said carbon nanotubes arein the form of aggregates having a macromorphology resembling a shapeselected from the group consisting of cotton candy, bird nests, combedyarn and open net aggregates.
 24. The method of claim 23, wherein saidaggregates have an average diameter of less than 50 microns.
 25. Amethod for producing a network of carbon nanotubes comprising the stepsof: (a) contacting carbon nanotubes with ozone at a temperature rangebetween 0° C. to 100° C. under conditions sufficient to formfunctionalized nanotubes which are greater in weight than said carbonnanotubes; (b) subjecting said functionalized nanotubes to conditionssufficient to cause crosslinking.
 26. The method of claim 25, whereinsaid temperature range is between 0° C. to 60° C.
 27. The method ofclaim 25, wherein said temperature range is between 20° C. to 50° C. 28.The method of claim 25, wherein said conditions sufficient to causecrosslinking include heating said functionalized nanotubes in air in atemperature range from 200° C. to 600° C.
 29. The method of claim 25,wherein said conditions sufficient to cause crosslinking include heatingsaid functionalized nanotubes in an inert atmosphere in a temperaturerange from 200° C. to 2000° C.
 30. A method for producing a network offunctionalized carbon nanotubes comprising the steps of (a) contactingcarbon nanotubes with ozone at a temperature range between 0° C. to 100°C. under conditions sufficient to form functionalized nanotubes whichare greater in weight than said carbon nanotubes; (b) treating saidfunctionalized nanotubes with a reactant suitable to react with moietiesof said functionalized nanotubes thereby adding at least a secondarygroup onto the surface of said functionalized nanotubes; (c) furthercontacting said nanotubes bearing secondary groups with an effectiveamount of crosslinking agent.
 31. The method of claim 30, wherein saidtemperature range is between 0° C. to 60° C.
 32. The method of claim 30,wherein said temperature range is between 20° C. to 50° C.
 33. Themethod of claim 30, wherein said crosslinking agent is selected from thegroup consisting of a polyol or polyamine.
 34. The method of claim 30,wherein said polyol is a diol and said polyamine is a diamine.
 35. Amethod for preparing a rigid porous structure comprising the steps of:(a) contacting carbon nanotubes with ozone at a temperature rangebetween 0° C. to 100° C. under conditions sufficient to formfunctionalized nanotubes which are greater in weight than said carbonnanotubes; (b) dispersing said functionalized nanotubes in a medium toform a suspension; and (c) separating said medium from said suspensionto form a porous structure of entangled functionalized nanotubes whereinsaid nanotubes are interconnected to form a rigid porous structure. 36.The method of claim 35, wherein said temperature range is between 0° C.to 60° C.
 37. The method of claim 35, wherein said temperature range isbetween 20° C. to 50° C.
 38. The method of claim 35, wherein said carbonnanotubes are in the form of aggregates having a macromorphologyresembling a shape selected from the group consisting of cotton candy,bird nests, combed yarn and open net aggregates.
 39. The method of claim35, further comprising heating said suspension in air to a temperaturein a range from about 200° C. to about 600° C. thereby forming saidrigid porous structure.
 40. The method of claim 35, further comprisingheating said suspension in an inert gas to a temperature in a range fromabout 200° C. to about 2000° C. thereby forming said rigid porousstructure.
 41. The method of claim 35, wherein said medium is water ororganic solvents.
 42. The method of claim 35, wherein said mediumcomprises a dispersant selected from the group consisting of alcohols,glycerin, surfactants, polyethylene glycol, polyethylene imines andpolypropylene glycol.
 43. The method of claim 35, wherein saidsuspension further comprises gluing agents selected from the groupconsisting of cellulose, carbohydrate, polyethylene, polystyrene, nylon,polyurethane, polyester, polyamides and phenolic resins.
 44. The methodof claim 35, further comprising the steps of: (a) forming said rigidporous structure into a mat; and (b) forming said mat into an electrode.45. An electrochemical capacitor having at least one electrodecomprising the functionalized carbon nanotubes prepared by the method ofclaim
 1. 46. An electrochemical capacitor having at least one electrodeprepared by a method which comprises the following steps: (a) contactingaggregates of carbon nanotubes with ozone at a temperature range between0° C. to 100° C. under conditions sufficient to form aggregates offunctionalized nanotubes which are greater in weight than said carbonnanotubes; (b) dispersing said aggregates of functionalized nanotubesprepared in step (a) in a liquid medium to form a slurry; (c) filteringand drying said slurry to form a mat of functionalized carbon nanotubes;and (d) subjecting said mat to conditions sufficient to cause thecrosslinking of said functionalized carbon nanotubes.
 47. The method ofclaim 46, wherein said temperature range in step (a) is between 0° C. to60° C.
 48. The method of claim 46, wherein said temperature range instep (a) is between 20° C. to 50° C.
 49. The electrochemical capacitorof claim 46, wherein said conditions of step (d) include heating saidmat to a temperature in the range of from 180° C. to 350° C.
 50. Anelectrochemical capacitor having at least one electrode formed by amethod comprising the following steps: (a) dispersing aggregates ofcarbon nanotubes in a liquid medium to form a slurry; (b) filtering anddrying said slurry to form a mat of carbon nanotubes; (c) treating saidmat with ozone at a temperature range between 0° C. to 100° C. underconditions sufficient to form a mat of functionalized carbon nanotubeswhich are greater in weight than said mat of carbon nanotubes.
 51. Ozonetreated carbon nanotubes which exhibit upon titration an acid titergreater than 2 meq/g.
 52. Ozone treated carbon nanotubes which exhibitupon titration an acid titer between 1.6 and 2.2 meq/g.
 53. Ozonetreated carbon nanotubes which exhibit upon titration an acid titerbetween 2.5 and 3.5 meq/g.
 54. An ozone treated carbon nanotubestructure which exhibits upon titration an acid titer greater than 1meq/g, said ozone treated carbon nanotube structure comprising amultiplicity of carbon nanotubes entangled with one another.
 55. Theozone treated carbon nanotube structure of claim 54, wherein saidstructure is in the form of aggregate of carbon nanotubes having amacromorphology resembling a shape selected from the group from thegroup consisting of cotton candy, bird nests, combed yam and open netaggregates.
 56. The ozone treated carbon nanotube structure of claim 54which substantially retains the original untreated carbon nanotubestructure.
 57. The ozone treated carbon nanotube structure of claim 54which exhibits upon titration an acid titer between 1 and 2 meq/g.
 58. Amethod for forming catalyst support comprising the steps of: forming anaggregate of carbon nanotubes, and contacting said aggregate with ozoneat a temperature range between 0° C. to 100° C. under conditionssufficient to form a functionalized aggregate which is greater in weightthan said aggregate.
 59. The method of claim 58, wherein saidtemperature range is between 0° C. to 60° C.
 60. The method of claim 58,wherein said temperature range is between 20° C. to 50° C.
 61. Themethod of claim 58, wherein said carbon nanotubes are multiwalled carbonnanotubes having a diameter of less than 0.1 micron.
 62. The method ofclaim 58, wherein said carbon nanotubes are single walled nanotubeshaving a diameter less than 5 nanometer.
 63. The method of claim 58,wherein the surface of said functionalized aggregate have an oxygencontent greater than 4 percent.
 64. The method of claim 58, wherein thesurface of said functionalized aggregate have an oxygen content greaterthan 6 percent.
 65. The method of claim 58, wherein said functionalizedaggregate exhibits upon titration an acid titer from 1 to 2 meq/g andretains the structure of said aggregate.
 66. The method of claim 58,wherein said functionalized aggregate exhibits upon titration an acidtiter from 1 to 2 meq/g.
 67. The method of claim 58, wherein saidfunctionalized aggregate exhibits a weight gain greater than 5% bycomparison to said aggregate.
 68. The method of claim 58, wherein saidfunctionalized aggregate exhibits a weight gain from 5% to 20% bycomparison with said aggregate.
 69. The method of claim 58, wherein saidfunctionalized aggregate exhibits a weight gain from 10% to 15% bycomparison with said aggregate.
 70. A catalyst support formed by themethod of claim
 58. 71. A catalyst support formed by the method of claim58 wherein the functionalized aggregate exhibits upon titration an acidtiter between 1 to 2 meq/g.
 72. A catalyst support formed by the methodof claim 58 wherein the functionalized aggregate exhibits upon titrationan acid titer greater than 1 meq/g and retains the structure of saidaggregate.
 73. A method for forming a catalyst support comprising thesteps of: forming a network of carbon nanotubes, and contacting saidnetwork with ozone at a temperature range between 0° C. to 100° C. underconditions sufficient to form a functionalized network which is greaterin weight than said network.
 74. The method of claim 73, wherein saidtemperature range is between 0° C. to 60° C.
 75. The method of claim 73,wherein said temperature range is between 20° C. to 50° C.
 76. Themethod of claim 73, wherein said carbon nanotubes are multiwalled carbonnanotubes having a diameter of less than 0.1 micron.
 77. The method ofclaim 73, wherein said carbon nanotubes are single walled nanotubeshaving a diameter less than 5 nanometer.
 78. The method of claim 73,wherein the surface of said functionalized network have an oxygencontent greater than 4 percent.
 79. The method of claim 73, wherein thesurface of said functionalized network have an oxygen content greaterthan 6 percent.
 80. The method of claim 73, wherein said functionalizednetwork exhibits upon titration an acid titer greater than 1 meq/g andretains the structure of said network.
 81. The method of claim 73,wherein said functionalized network exhibits upon titration an acidtiter from 1 to 2 meq/g.
 82. The method of claim 73, wherein saidfunctionalized network exhibits a weight gain greater than 5% bycomparison to said network.
 83. The method of claim 73, wherein saidfunctionalized network exhibits a weight gain from 5% to 20% bycomparison with said network.
 84. The method of claim 73, wherein saidfunctionalized network exhibits a weight gain from 10% to 15% bycomparison with said network.
 85. A catalyst support formed by themethod of claim
 73. 86. A catalyst support formed by the method of claim73 wherein the functionalized network exhibits upon titration an acidtiter between 1 to 2 meq/g.
 87. A catalyst support formed by the methodof claim 73 wherein the functionalized network exhibits upon titrationan acid titer greater than 1 meq/g and retains the structure of saidnetwork.
 88. A method for forming a catalyst support comprising thesteps of: forming a rigid porous structure comprising carbon nanotubes,and contacting said rigid porous structure with ozone at a temperaturerange between 0° C. to 100° C. under conditions sufficient to form afunctionalized rigid porous structure which is greater in weight thansaid rigid porous structure.
 89. The method of claim 88, wherein saidtemperature range is between 0° C. to 60° C.
 90. The method of claim 88,wherein said temperature range is between 20° C. to 50° C.
 91. Themethod of claim 88, wherein said carbon nanotubes are multiwalled carbonnanotubes having a diameter of less than 0.1 micron.
 92. The method ofclaim 88, wherein said carbon nanotubes are single walled nanotubeshaving a diameter less than 5 nanometer.
 93. The method of claim 88,wherein the surface of said functionalized rigid porous structure havean oxygen content greater than 4 percent.
 94. The method of claim 88,wherein the surface of said functionalized rigid porous structure havean oxygen content greater than 6 percent.
 95. The method of claim 88,wherein said functionalized rigid porous structure exhibits upontitration an acid titer greater than 1 meq/g and retains the structureof said rigid porous structure.
 96. The method of claim 88, wherein saidfunctionalized rigid porous structure exhibit upon titration an acidtiter from 1 to 2 meq/g.
 97. The method of claim 88, wherein saidfunctionalized rigid porous structure exhibit a weight gain greater than5% by comparison to said rigid porous structure.
 98. The method of claim88, wherein said functionalized rigid porous structure exhibit a weightgain from 5% to 20% by comparison with said rigid porous structure. 99.The method of claim 88, wherein said functionalized rigid porousstructure exhibit a weight gain from 10% to 15% by comparison with saidrigid porous structure.
 100. A catalyst support formed by the method ofclaim
 88. 101. A catalyst support formed by the method of claim 88wherein the functionalized rigid porous structure exhibit upon titrationan acid titer between 1 to 2 meq/g.
 102. A catalyst support formed bythe method of claim 88 wherein the functionalized rigid porous structureexhibits upon titration an acid titer greater than 1 meq/g and retainsthe structure of said rigid porous structure.